Process for production of biodiesel from high acid feed

ABSTRACT

Biodiesel fuels are produced efficiently and at low cost from waste oil feeds containing high concentrations of fatty acids. By first producing an ester of a water immiscible alcohol, water of reaction from the esterification can be easily stripped out with and separated from the alcohol, which is recycled to the esterification reaction. Subsequent transesterification with glycerin produces a feed with a sufficiently low acid value to allow methanolysis using a basic catalyst to proceed rapidly without consumption of large quantities of catalyst or organic acid salt formation.

FIELD OF THE INVENTION

The current process relates to the field of the production of biodiesel fuels by esterification of fatty acids contained in waste oil feeds. Specifically, the present invention relates to a process for efficient production of biodiesel fuels using waste oil feeds containing high concentrations of free fatty acids.

BACKGROUND OF THE INVENTION

For processes that produce fatty acid methyl esters (FAME) for biodiesel fuel or other purposes, the less expensive feeds such as yellow grease typically contain 15% or more free fatty acid or salts of fatty acids that can be liberated by acidulation. In order to produce the methyl or other alkyl esters via the preferred base-catalyzed transesterification process, these free fatty acids must be esterified with an alcohol or polyol, and the water of reaction must be removed to very low concentrations to drive the reversible reaction to nearly complete conversion of the acids. Otherwise, consumption of the base, typically sodium or potassium methoxide, will be excessive as will organic acid salt formation.

One common approach to the esterification is to do it using methanol since this produces the methyl ester wanted in the eventual end product. However, the water of reaction azeotropes methanol with it, and since they are totally miscible, the methanol must be distilled away from the water. An alternative is molecular sieve drying, which entails periodic regeneration. Both represent a significant addition of equipment and require additional energy input, especially relative to a simple decanter. This is a major drawback to this route. Another drawback of this process is the need to run higher pressures with methanol at a given temperature. Some claim advantageous methods to circumvent this issue, but they tend to use a high amount of mineral acid and ignore the final disposition of the water of reaction, for example U.S. Pat. No. 6,696,583 and published PCT application WO 0228811. The same omission occurs in U.S. Pat. No. 4,698,186, which teaches using an acidic ion-exchange resin as the pre-treatment catalyst, which thus entails an investment in a resin vessel, the resin itself, and a falling film evaporator. Alternatively, U.S. Pat. No. 5,434,279 teaches starting at low free fatty acid levels, which then precludes the use of the low-cost feed materials.

Numerous references to the use of glycerin in biodiesel production appear in the prior art, for example in U.S. Pat. No. 4,698,186. Further, U.S. Pat. No. 6,822,105 discloses pretreating a FAME process feed by esterifying with glycerin. The expressed surprise of better results with using crude glycerin is readily explained by the significant level of methanol in that material azeotroping out the water of reaction. However, there is a significant penalty of lost methanol from doing this. Most notable though is that at best the process reported in U.S. Pat. No. 6,822,105 requires about 7 hours of batch time to get the desired low level of acidity before proceeding to the methanolysis.

Another reference (Trans Am Soc. Of Agricultural Eng., Vol. 46 (4) (945-954)) describes a rather complicated pretreatment process, which still leaves the methanol/water separation for later.

Thus there has remained a need for an efficient, low-investment process for converting high fatty acid content feeds to biodiesel fuel. The present invention fills this need by providing such as process.

SUMMARY OF THE INVENTION

Feeds comprising glycerides of fatty acids and containing>1% free fatty acids, going into processes for making methyl esters of fatty acids (FAME), such as biodiesel, are first subjected to esterification with an alcohol that is substantially immiscible with water. The water of reaction is vaporized along with the alcohol and a simple phase separation removes the water from the system virtually free of organics. After most of the esterification has occurred, glycerol is added in molar excess and a transesterification of the alkyl ester to glycerides is effected by evaporating out the alcohol via inert-gas stripping or vacuum stripping in the vessel. Some of the higher alkyl ester may intentionally be left in the mix to enhance the properties of the final biodiesel product, such as cetane value. The resulting intermediate glyceride product with low acidity is then subjected to any common base-catalyzed transesterification with a C₁ to C₄ alcohol to produce a corresponding alkyl ester product and a byproduct glycerol (glycerin) stream. In this manner, a feed of relatively high free fatty acid content can be efficiently used to produce biodiesel at mild conditions using simple equipment in relatively short batch or residence times with excellent yields and improved properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Illustrates an exemplary embodiment of the process according to the current invention.

FIG. 2 Is a graph illustrating the rapid reduction in acid value obtained by the process according to the current invention.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of the process will now be described with reference to FIG. 1. The following describes a preferred embodiment of the invention, but many variations can be envisioned. Parameters subject to optimization in the process include the molar excesses of water immiscible alcohol and glycerol (glycerin) employed, and the use and timing of applying vacuum. This design is for a batch mode of operation, but it could readily be adapted into continuous process by utilizing multiple vessels in series.

The description that follows pertains to processing beginning with feeds containing high levels of free fatty acids as one is likely to encounter in using waste oils and greases. The feeds comprise glycerides of fatty acids, primarily triglycerides, and contain at least 1 weight percent of free fatty acids. Some feeds, for example yellow grease contain up to 15 weight percent, and considerably more is common in other waste oils and greases. All of these feeds are considered to be within the scope of the invention. The free acid content of the feed is typically expressed as acidity as measured by the acid value (A.V.) of the feed, determined by titration. Acid value is measured herein by titration with a 0.1 N NaOH solution and thymol blue indicator (yellow to blue). U.S. Pat. No. 6,822,105 reports typical acid values for a number of feeds, measured using a nearly identical titration method, using KOH instead of NaOH.

The exemplary embodiment described is for the production of fatty acid methyl esters (FAME) since this is the ester typically used for Biodiesel production. However, it should be noted that the present invention is not limited to the production of methyl esters of fatty acids. While the methyl ester is preferred the process of the present invention is equally suitable for the production of other alkyl esters of fatty acids including, but not limited to; ethyl, propyl and butyl.

The below described exemplary embodiment assumes an A.V. of approximately 32 for the feed oil. It is presumed that the feeds have been physically cleaned of debris and do meet certain modest specifications. The feed oil is pumped in from a tank farm into a measuring tank not shown where it is preheated as well as measured. The feed is drained into the esterification reactor 12, and additional heating is begun. A water immiscible alcohol is added at about a 20-100% molar excess to the estimated amount of free fatty acid, typically the alcohol will make up about 10 wt % of the charge. Preferred alcohols include hexanol, heptanol, 2-ethylhexanol (2EH) and isononanol. To this an acidic catalyst, such as toluene-sulfonic acid or sulfuric acid at about 0.1 wt. % is added. This will add acidity about equivalent to an A.V. of 0.3 (red to yellow). The mix is heated to about 180° C., with nitrogen sparging.

The alcohol reacts with the free fatty acid to make an ester and forming water in the process. This water must be removed for the reaction to progress to a high level of completion. This is done by evaporation and boiling of water and alcohol from the reactor 12 into a circulating nitrogen stream 14. The vapors are condensed overhead in condenser 16, and the water separates below an alcohol phase in the separator 18, with the alcohol refluxing back to the reactor to maintain the excess alcohol. After about 2 to 2.5 hours, the acid value should be below 6 (>80% reacted). At this point, refluxing of the alcohol is discontinued with the alcohol instead being collected in a small holding tank 26. Stored glycerin byproduct stripped of alcohol is now added to the reactor 12 in an equal mole ratio to the initial alcohol charge. The intent is to strip out the bulk of the alcohol and have the glycerol displace the alcohol on the fatty acid ester by transesterification. Further reduction of the A.V. occurs as this goes on to get the A.V. below 1.0, which is about 0.5 wt % fatty acid left.

The pressure on the reactor may be run slightly negative to enhance alcohol stripping with as much nitrogen flow as possible to enhance removal of the alcohol. The 2-ethylhexanol fatty acid ester has an exceptionally high cetane rating (G. Knothe et al., Fuel 82 (2003) 971-975) and should also improve the cloud point of the biodiesel (J. Am Oil Chem Soc (1997) 74(8):951-5). Hence, it is only an economic consideration on how much should be stripped out, and the amount left in can be adjusted as desired for final product properties. It can be expected to take 1 to 1.5 hours for this portion of the esterification step, and preferably the stripping will continue in the subsequent transesterification vessel 22 under vacuum. About 3.5 hours total for the esterification step will be typical.

While the esterification is ongoing, a catalyst solution of dry caustic (NaOH or KOH) and methanol is made up in a catalyst premix vessel 24. As an alternative, it can be purchased in liquid form from suppliers like BASF, Evans City, Pa. A simple stirred vessel can be converted to the transesterification reactor by adding a methanol feed line and meter, an overhead condenser, and capability to run up to 20 psig and full vacuum. The esterification reactor product is flashed as it is being transferred into an evacuated transesterification vessel 22 kept at about 40 mm Hg absolute. This pressure and the adiabatic temperature are maintained for up to an hour to remove as much water immiscible alcohol as possible and drive the glycerol transesterification further. The overhead vapors are condensed, and drop via a barometric leg back into the alcohol surge tank 26. Next, the vessel contents are cooled to about 70° C. This can be attained by cooling water in internal coils in the vessel or circulating through an external cooler. The sodium methoxide catalyst solution can be added to the transesterification vessel 22 during cooling or afterwards. Methanol, fresh plus any recycle, is added at about 15 weight percent of the pretreated oil feed. The transesterification reaction is run with agitation at about 66° C. and the pressure is permitted to go as high as 20 psig.

As the transesterification reaction proceeds, glycerin byproduct forms. It is much denser than the fatty acid methyl esters (FAME), and essentially insoluble in the FAME. Removing it from the reactor contents should aid the extent of the reaction in the forward direction. While not essential, a recirculating bottoms stream is sent through a “hydroclone” device 28, which uses centrifugal force to concentrate the denser material in the bottom cone section. These bottoms can continuously or intermittently flow into the crude glycerin collection tank. Doing this should ensure essentially complete conversion to the FAME in less than two hours, and it decreases how much gravity separation must be done in the subsequent vessel.

Once the transesterification according to the current invention is complete, the material may be rapidly pumped over to a settler/washer vessel 30. There is an option for some sulfuric acid to be added while this is going on, to do a partial neutralization of the caustic catalyst. Once transfer is complete, the material is allowed to sit undisturbed for about one hour to allow additional separation of the heavier glycerin. The bottoms are then drained or pumped from the boot to the crude glycerin hold tank 36. This should achieve removal of most of the glycerol, excess methanol, and salts from the biodiesel.

Next, the FAME may be water washed by adding recycle and fresh warm wash water. After agitating, the contents are allowed to settle undisturbed for about an hour. This, and perhaps a second water wash, should remove most of the remaining methanol, glycerol, and sodium or potassium salts. The bottom phase is pumped from the boot to a wash water hold tank 38. Following this, the rest of the contents are pumped or drained from the bottom off to the side above the boot into the stripper feed tank 32.

From the oversized stripper feed tank 32, material is continuously pumped to the top of stripping column 34 after being heated to about 100° C. The temperature and vacuum at the top of the stripper remove residual traces of methanol, water, and other lighter material. The bottoms are pumped through filters, and adsorption columns as needed to remove particulates or other trace impurities. The product is held in alternating product receivers 40 and 42 for checking of the many specifications on the biodiesel. In-spec product is then pumped to a storage tank.

EXAMPLE

An experiment run on the laboratory scale demonstrates the significant time improvement in reducing the reaction mixture acidity, and thus driving the esterification reactions to completion. This lab apparatus lacked the capability to operate under vacuum; hence, the preferred mode of operating could not be fully demonstrated. The experiment was run in the 1.5 liter resin-kettle reactor. The feed was synthesized by adding purchased oleic acid to common supermarket vegetable oil.

To 602 grams of soybean oil was added 106.1 grams of oleic acid and 98 grams of 2-ethylhexanol. The starting acid value (A.V.) of this mixture was approximately 32. 794.2 grams of this reaction mixture were charged to the reactor, which was heated above 160° C., and sparged with nitrogen at about 1 liter per minute rate.

The reaction was attempted initially using a tin catalyst (Fascat 2001), but when it was noted that reaction water was not appearing after an hour at temperature, a small amount of toluene sulfonic acid was added at about the 68-minute point. This represents the starting point of the method being demonstrated. As can be seen on the chart in FIG. 2, it took about 3.5 hours then to go from an acid value of 21 to less than one. Notably, this is less than half the time reported in U.S. Pat. No. 6,822,105. The mineral acid catalyst contributed about another 0.3 or less to the overall acidity. These levels of acidity would consume a small percentage of the NaOH or KOH catalyst used in the subsequent methanolysis reaction to make biodiesel.

There has thus been demonstrated an efficient process for conversion of a high fatty acid content feed to biodiesel fuel. The benefits of the current process include: relatively rapid esterification of free fatty acids to glycerides; simple, clean separation of water of reaction; effective catalyst concentration with negligible contribution to cost or acidity; a built in opportunity to improve cetane rating; and a process which uses simple and commonly available equipment. 

1. A process for producing biodiesel comprising, providing a feed stream comprising glycerides of at least one fatty acid and at least 1 weight percent of free fatty acids; adding a water immiscible alcohol to the feed stream to form a first reaction mixture, wherein the alcohol is added in a molar excess relative the amount of free fatty acids in the feed stream; forming an ester of the free fatty acids and alcohol by heating the first reaction mixture in the presence of an acidic catalyst and stripping water and some of the water immiscible alcohol from the first reaction mixture; adding glycerol to the ester thus produced to form a second reaction mixture; forming glycerides of the free fatty acids by heating the second reaction mixture and stripping the water immiscible alcohol from the second reaction mixture; adding at least one C₁-C₄ alcohol to the glycerides thus produced to form a third reaction mixture; forming an ester of the fatty acids and at least one C₁-C₄ alcohol, and a glycerol byproduct, by heating the third reaction mixture in the presence of a catalyst comprising a sodium or potassium alkoxide of the at least one C₁-C₄ alcohol.
 2. The process according to claim 1, wherein the water immiscible alcohol is added to the feed stream in a 20 to 100 percent molar excess relative to the amount of free fatty acids in the feed stream.
 3. The process according to claim 1, wherein the glycerol is added to the first reaction mixture in an equimolar or greater amount relative to the amount of water immiscible alcohol added to the feed stream.
 4. The process according to claim 1, further comprising sparging the first reaction mixture and second reaction mixture with nitrogen.
 5. The process according to claim 1, wherein the step of forming a glyceride of the free fatty acids is conducted so that a portion of the ester from the first reaction mixture remains in the second reaction mixture.
 6. The process according to claim 1, further comprising sparging the first reaction mixture with nitrogen and heating the second reaction mixture under vacuum such as to vaporize out most of the alcohol, and condensing the alcohol for recycle.
 7. The process according to claim 1, wherein the water immiscible alcohol is hexanol, heptanol, isononanol, or 2-ethylhexanol.
 8. The process according to claim 1, further comprising separating a bottom stream from the third reaction mixture comprising the glycerol byproduct and recycling the glycerol byproduct to the second reaction mixture either as is or following purification.
 9. The process according to claim 1, wherein the stripped water and water immiscible alcohol are separated after stripping and the stripped water immiscible alcohol is returned to the first reaction mixture. 